The present invention relates to an objective lens driving device for use in an optical disk apparatus, and more specifically to an objective lens driving device featured in location of a coil and a magnet which constitute a compensating mechanism for a tilt in a radial direction of an optical disk medium.
Generally, in an optical disk apparatus, a laser light emitted from a light source typified by a semiconductor laser is condensed and irradiated onto an information recording surface of the optical disk medium in order to record, reproduce or erase information.
In this optical disk medium, a track position in a recording surface on which the information is recorded, ceaselessly displaces in a focusing direction corresponding to a direction perpendicular to the plane of the optical disk medium and in a tracking direction corresponding to a radial direction of the optical disk medium, because of a surface swing and a eccentricity.
Therefore, a conventional optical disk apparatus is required to drive the objective lens to an optimum position in accordance with a desired track. For this purpose, the conventional optical disk apparatus is so constructed that the objective lens is driven in the focusing direction and in the tracking direction by an objective lens driving device.
Recently, with an increased amount of information processed in a computer, the recording amount required in the optical disk has the tendency of increasing. For the increased recording amount of the optical disk, a recording density can be increased by reducing the condensed beam diameter of the laser light irradiated onto the information recording medium. In addition, the reduction of the beam diameter can be realized by shortening the wavelength of a laser light source and by increasing the NA (numerical aperture) of the objective lens.
Here, in the case that the high-density large-capacity recording is realized by increasing the NA of the objective lens, when information is recorded to, or erased or reproduced from the optical disk medium, the margin of tilt of the optical disk medium to a light axis of the laser beam condensed by the objective lens will become small, with the result that it becomes necessary to compensate the objective lens for a tilt of the optical disk medium in a radial direction.
Therefore, the objective lens driving devices becomes required to have a mechanism for driving the objective lens not only conventionally in the focusing direction and in the tracking direction but also in a radial-tilt direction.
A simple means for enlarging the information recording area in the optical disk medium having the same shape and size, enables to the recording capacity of the optical disk medium, even if it is slight. In this case, a size of a light pickup is particularly minimized in the radial direction in order to access an innermost area of the optical disk medium.
Furthermore, recently, an optical disk apparatus has been proposed which is so configured to record, reproduce and erase recording medium having different thicknesses such as CD (Compact Disk) and DVD (Digital Versatile Disk), by use of one objective lens and two laser sources.
In this case, since the height of the information recording surface and the thickness of the cover glass vary from one disk to another, it is necessary to cause the condensed point, namely, the focus of the light beam from the laser light source to coincide the information recording surface of each disk, by shifting the position of the objective lens in the focusing direction.
In addition, such disks having a poor mechanical characteristics which causes a large surface swing of the optical disk medium, or has a large eccentricity or tilt of the optical disk medium, start to appear with popularization of use of optical disks.
In order to obtain a good characteristics of information recording, reproducing and erasing for those various optical disk media, an objective lens driving device has become necessary, which can easily realize the tilt control, with less variation in a tilt movement sensitivity or a tilt movement linearity, even in the condition that the objective lens has been shifted in the focusing direction or in the tracking direction.
Under the above mentioned circumstance, recently, an objective lens driving device has become strongly demanded, which can drive the objective lens in three directions of the focusing direction, the tracking direction and the radial tilt direction, and which has less variation in a tilt movement sensitivity or a tilt movement linearity even if the objective lens has been shifted in the focusing direction or in the tracking direction, and which has a minimized size in a radial direction.
One example of objective lens driving devices capable of compensating the radial tilt of the optical disk medium is disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-09-231595, entitled “LENS ACTUATOR WITH TILT CORRECTION”.
Namely, as shown in the exploded perspective view of FIG. 9, this lens actuator with tilt correction, includes tilt coils 25 provided on opposite side surfaces of an objective lens holder 21, respectively, and magnets 26 and 27 of opposite polarities provided at positions opposing each tilt coil 25, thereby to enable a tile movement of an objective lens 23 in the radial direction.
In brief, as shown in FIG. 9, the objective lens holder 21 has the objective lens 23 mounted at a center position thereof, and a pair of slits 22 formed at both sides of the objective lens 23. Driving systems for the lens actuator are shown by arrows designated by “F” (focusing axis), “R” (tracking axis) and “T” (tangent axis of the optical disk).
The objective lens holder 21 includes a pair of rectangular flat coils 24 provided on each of opposite side surfaces (four in total) for a tracking direction driving. As a coil for the tilt correction, the rectangular flat coils 25 are provided on opposite side surfaces in an optical disk radial direction (R). In addition, the elongated magnets 26 and 27 of opposite polarities to each other are provided on each of side yokes 33 and 34, which will be explained hereinafter, at positions corresponding to an upper side and a lower side of the rectangular flat coil 25.
For a magnetic circuit of the lens actuator, the yokes 33 and 34 are abutted to an actuator base 28 so that a substantially closed magnetic circuit is constituted by magnets 31 and 32, for the driving in the focusing direction and in the tracking direction.
On opposite side surfaces of the actuator base 28, there are provided the side yokes 33 and 34 of a C-shape in a plan view, for a tilt adjustment driving of the lens holder. The above mentioned magnets 26 and 27 are bonded to the side yokes 26 and 27.
Incidentally, a printed circuit board (not shown) is bonded to the objective lens holder 21 through copper foil portions 35 and 36 so that the printed circuit board is supported.
Similarly, rectangular printed circuit boards 37 and 38 are bonded to the actuator base 28 through copper foil portions 39 and 40. Four spring wires 41 made of phosphor bronze are fixed to the printed circuit boards located at opposite ends of the spring wires, thereby resiliently supporting the objective lens holder 21.
This lens actuator is driven in such a manner that an electric current is supplied to the pair of rectangular flat coils 25 in the same direction, so that the generated magnetic fields are in bilateral symmetry, with the result that the magnetic driving forces of the respective coils have different directions at opposite sides of the objective lens holder 21 under the Fleming's left hand rule, to rotate the objective lens holder 21 around its center of gravity or a center point of support, thereby to be enable the tilt correction of the optical disk (not shown).
In the above mentioned conventional objective lens driving device, in the condition that the objective lens has been shifted in the tracking direction or in the focusing direction, no attention has been paid for ensuring the sensitivity and the linearity of the tilt movement in the radial direction. When the objective lens has been shifted in the tracking direction or in the focusing direction, the sensitivity and the linearity of the tilt movement change from those when the objective lens is in its neutral position, with the result that it is not possible to stably control the objective lens to a desired position, and therefore, the characteristics of the information recording, reproducing and erasing become deteriorated.
Furthermore, the objective lens driving device has a large size in the radial direction because the tilt coils provided at the side surfaces of the lens holder and the tilt magnets provided at the surfaces opposing the tilt coils, so that an accessible range to a center portion area of the optical disk medium is restricted, with the result that it is not possible to access an innermost recording area of the optical disk.
With an increased data transfer rate, the number of revolution of the optical disk medium has the tendency of increasing. When a primary resonance frequency in the tilt direction becomes coincident with a rotational frequency of the optical disk medium, a rolling mode resonance phenomenon will occurs.